Wierd Audi Symptom

[tturse@ps-b.com (Thomas Turse)]

One more perplexing phenomenon that has been occurring to my 85 4ksq
recently.  Driving up the same stretch of up hill highway (I70 in CO, from
Morrison to Buff Bills Grave exit)  the car seems to exhibit varying levels
of performance.  Sometimes I can stay in 5th gear and easily get up the
entire hill at 80-85 mph but other times the car will seem to just lose
momentum w/ no power to hold or increase speed- the speed drops to about 60
unless I downshift into 4th.  In 4th it will do 85mph but at 5000 rpm.  I
drive this route into Denver every day and have tried to consider some
possible non-mechanical problems.  I've considered the heat/defrost being on
and off, gas tank full or empty, wind direction/speed, fuel octane, etc.. w/
no noticeable changes.  Sometimes the car just ripps up the mountain in 5th
gear and other times in just dogs.  The engine is really strong.  I
initially thought was just a combination of the gearing:hp:weight ratio, but
this is a pretty drastic change.
Any thoughts would be very much appreciated.

Sincerely,

Thomas C.  Turse
Evergreen, CO
tturse@ps-b.com